capacitance:capacitor:defibrillator:dielectric strength:dielectric:electric potential:electron volt:equipotential line:grounding:mechanical energy:Figure 19.25Automated external defibrillators are found in many public places. These portable units provide verbal instructions for use in the important first few minutes for a
person suffering a cardiac attack. (credit: Owain Davies, Wikimedia Commons)Example 19.11 Capacitance in a Heart Defibrillator
A heart defibrillator delivers4.00×10^2 Jof energy by discharging a capacitor initially at1.00×10^4 V. What is its capacitance?
StrategyWe are givenEcapandV, and we are asked to find the capacitanceC. Of the three expressions in the equation forEcap, the most
convenient relationship is
(19.77)Ecap=CV
2
2
.
SolutionSolving this expression forCand entering the given values yields
(19.78)
C =
2 Ecap
V^2
=
2(4.00× 102 J)
(1.00× 104 V)^2
= 8.00× 10 – 6F
= 8.00 μF.
DiscussionThis is a fairly large, but manageable, capacitance at1.00×10^4 V.
Glossary
amount of charge stored per unit volta device that stores electric chargea machine used to provide an electrical shock to a heart attack victim's heart in order to restore the heart's normal rhythmic patternthe maximum electric field above which an insulating material begins to break down and conductan insulating materialpotential energy per unit chargethe energy given to a fundamental charge accelerated through a potential difference of one volta line along which the electric potential is constantfixing a conductor at zero volts by connecting it to the earth or groundsum of the kinetic energy and potential energy of a system; this sum is a constant688 CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
This content is available for free at http://cnx.org/content/col11406/1.7